Remember these are the people that were advocating insurgency (click here for post). It seems to me that these people are so off their rocker that an attempt on the President is, at least, plausible and, in some (many?) of their minds justified. They are really detached from reality.
That is one thing I honestly do worry about, when W gets 4 more years. These people are so hate-crazed that they might try anything. After all, what kind of people write a book about thinking up ways to assassinate a president?